1. Field of the Invention
The present invention relates to electron-beam probing methods and systems useful, for example, in analyzing operation of Very Large Scale Integrated (VLSI) circuit devices. In particular, the present invention relates to electron-beam probing methods and systems in which test patterns generated to exercise a device are synchronized with pulsed-laser photoemission of an electron probe beam.
2. The Prior Art
Characterizing and verifying the operation of VLSI circuit devices is an important element of their design. Probing internal nodes of such devices plays an increasingly significant role in this process.
The recent development of electron-beam (E-beam) probe tools and techniques has greatly assisted in overcoming the problems involved in probing internal nodes of integrated circuits. E-beam probing uses the principle of voltage contrast in a scanning electron microscope (SEM) to perform VLSI diagnosis through non-contact electron-beam measurement of internal signals in a VLSI device. A focused beam of primary electrons is directed toward a conductor within a circuit specimen as signals are applied to the specimen. Detected secondary electrons are indicative of the surface electrical potential on conductors within the specimen. See, for example, E. Menzel & E. Kubalek, Fundamentals of Electron Beam Testing of Integrated Circuits, 5 SCANNING 103-122 (1983), and E. Plies & J. Otto, Voltage Measurement Inside Integrated Circuit Using Mechanical and Electron Probes, IV SCANNING ELECTRON MICROSCOPY 1491-1500 (1985).
Commercial introduction by Schlumberger in 1987 of the "IDS 5000.TM." workstation-based, electron-beam test probe system greatly simplified E-beam probing of circuit chips and increased the efficiency of circuit debug. See S. Concina, G. Liu, L. Lattanzi, S. Reyfman & N. Richardson, Software Integration in a Workstation Based E-Beam Tester, INTERNATIONAL TEST CONFERENCE PROCEEDINGS (1986); N. Richardson, E-Beam Probing for VLSI Circuit Debug, VLSI SYSTEMS DESIGN (1987); S. Concina & N. Richardson IDS 5000: an Integrated Diagnosis System for VLSI, 7 MICROELECTRONIC ENGINEERING (1987); and see U.S. Pat. Nos. 4,706,019 and 4,721,909 to N. Richardson, which are incorporated herein by reference. See also J. FROSIEN, et al., High Performance Electron Optical Column for Testing ICs with Submicrometer Design Rules, MICROELECTRON. ENG. (NETHERLANDS), vol. 7., no. 2-4, pp. 163-72 (First European Conference on Electron and Optical Beam Testing of Integrated Circuits, Grenoble, France, Dec. 9-11 , 1987).
FIG. 1 is a block diagram of a prior art electron-beam test probe system 110. The system has three main functional elements: an electron-beam test probe 112, a circuit exerciser 114, and a data processing system 116 which includes a display terminal 118. The circuit exerciser 114 may be a conventional integrated circuit tester, such as a model "S 15.TM." tester available from Schlumberger Technologies of San Jose, Calif., which can repeatedly apply a pattern of test vectors to the specimen circuit over a bus 124. The specimen circuit (device under test, or DUT) 126 is placed in a vacuum chamber 128 of the electron-beam test probe 112 so that potential measurements can be made as the test vector pattern is applied. The points at which such measurements are to be made are sent to the electron-beam test probe 112 by the data processing system 116 over a bus 122. The data processing system 116 may also be used to specify the test signal pattern used and the timing of the potential measurements relative to the test signal pattern. The electron-beam test probe system is controlled by an operator who inputs commands through the display terminal 118.
SEMs used for electron-beam probing, such as that in the IDS 5000 system, are equipped with high-speed beam pulsing hardware sometimes referred to as a "beam-blanker." An example of such hardware is described in U.S. Pat. No. 4,721,909. Directing a pulsed electron beam at a particular node of interest provides a mode of operation much like that of a sampling oscilloscope, in which images can be produced of waveforms at one or more nodes in the specimen circuit as test vector patterns are applied to the specimen circuit.
For each point of the waveform image, a measurement is made by pulsing the electron beam at a specific time during application of the test vector pattern to the specimen circuit. Since the time needed to make a potential measurement is generally longer than the time over which the test signal pattern remains constant, stroboscopic techniques are used. That is, the electron beam is turned on for a brief period of time at a point in the test signal pattern. Each time the electron beam is so pulsed, a measurement of the potential on a node of the specimen circuit is made.
Since a single measurement has insufficient statistical accuracy to allow an accurate determination of the potential, measurements made over many repetitions of the test vector pattern are averaged. For even a relatively simple circuit under test, tens of thousands or even hundreds of thousands of repetitions of the test vector pattern may be needed to acquire the data represented in a waveform image. Coordination of the electron-beam pulses with the test vector pattern may be effected by a trigger generator circuit of the electron-beam test probe system under control of data processing system 116.
New design and process technologies result in ever-faster VLSI circuit devices having internal elements with decreased switching transition times. Device characterization and verification thus become increasingly difficult. Conventional electron-beam probe systems of the kind described above have at least two limitations which become critical as switching transition times of VLSI circuit devices decrease.
First, the electron source brightness obtained with the conventionally-used tungsten or lanthanum hexaboride thermionic emitters is low; at the necessarily low beam accelerating voltage of around 1 kV, the amount of peak current in the final electron probe is limited to about 1 nA when the probe diameter is around 0.1 micron. When the beam is pulsed with a duration of less than 50 ps (picoseconds) in order to perform high speed sampling measurements, the statistical average number of electrons per pulse is less than one. This leads to intractable signal processing problems and intolerably long signal integration times. For example, as the duty cycle (the trigger period divided by the beam pulse width) increases, leakage currents and other measurement limitations result in degraded measurement accuracy. Data acquisition time is a real concern, particularly with the lengthy test vector patterns require to exercise ever more complex integrated circuits. Second, and because of the relatively low beam accelerating voltage, the construction of an effective beam pulser/blanker to produce pulses of duration much less than 50 ps is extremely difficult, and may be a practical impossibility in a commercial machine. Thus, even with a brighter source than those mentioned above, routinely generating pulses of less than 50 ps would still be difficult. Though brighter sources such as thermal field emitters are known (see, for example, L. SWANSON, A Comparison of Schottky and Cold Field Emission Cathodes (FEI Company, Beaverton, Oreg., January 1989), they are not routinely used because of the extremely stringent requirements on the operating vacuum pressure around the source.
Work done at the IBM Watson Research Center, Yorktown Heights, N.Y., shows that it is possible, with a laser stimulated photocathode, to extend the performance of an electron beam prober to the point where beam pulses can be generated which are of less than 2 ps duration and which have more than 2000 electrons per pulse. See, for example, P. MAY, et al., Picosecond photoelectron scanning electron microscope for noncontact testing of integrated circuits, APPL. PHYS. LETT. 51 (2), pp. 145-147 (1987); P. MAY, et al., Noncontact High Speed Waveform Measurements with the Picosecond Photoelectron Scanning Electron Microscope, IEEE J. QUANTUM ELECTRON. (USA), Vol 24., No. 2, pp. 234-239 (February 1988); J-M. HALBOUT, et al., SRAM Acess Measurements Using a Picosecond Photoelectron Scanning Electron Microscope, Paper No. WPM 8.2, IEEE International Solid State Circuits Conference (ISSCC, 1988); P. MAY, et al., Waveform Measurements in High Speed Silicon Bipolar Circuits Using a Picosecond Photoelectron Scanning Electron Microscope, IEEE International Electron Devices Meeting (IEDM, 1987). pp. 92 et seq.
While a pulsed-laser photoemission source offers a dramatically brighter source of short-duration electron-beam pulses (on the order of 10.sup.1 electrons per pulse vs. 10.sup.-2 electrons per pulse with typical thermionic sources), the pulsed-laser electron-beam cannot be readily turned on and off in response to trigger pulses from the test pattern generator in the manner employed with thermionic sources. Nor can the phase of the pulsed-laser electron-beam be readily shifted with respect to a fixed time reference. Thus, matching the electron-beam pulses to the test pattern from the circuit exerciser by controlling the electron-beam source does not appear feasible with a pulsed-laser photoemission source.
A further problem is raised by the fact that switching times of elements within the device under test (DUT) can be below 10 ps, yet conventional techniques of connecting to the device do not allow signals with transition times much below 50 ps to be effectively coupled into or out of the device. In general, a complex component is made by interconnecting thousands of active elements together on a chip. What is of interest is the actual performance of elements internal to the device. The non-contact, high-bandwidth measurement technique of electron-beam probing allows detailed observation of the timing relationships of signals internal to the device.
In order to make the most accurate, low-jitter measurements, the bandwidth of the connection between the test pattern generator and the device must be as high as possible to minimize degradation of switching edge speeds. Even then, each device connection (e.g., bond wire, probe pin and even bond pad) represents an unknown, complex impedance between the stimulus source and the on-chip element. Exact knowledge of the waveforms at the output of the test pattern generator does not therefore guarantee knowledge of the waveforms inside the device at the element of interest. By way of illustration, FIG. 2 shows schematically a portion of a VLSI circuit device 30 mounted on a carrier 32. Active elements forming part of device 30 include flip-flops 34 and 36 and inverters 38 and 40. Each of flip-flops 34 and 36 has a data input port D and a clock input port C. Bond wire 42 electrically connects bond pad 44 of device 30 to bond pad 46 of carrier 32. Bond pad 46 is in turn electrically connected to an input pin 48. Circuit exciser 14 causes the electron-beam probe to be pulsed in stroboscopic fashion by sending trigger pulses (as shown in line "a" of FIG. 3) to the test probe 12. Test patterns produced by circuit exerciser 14 are synchronized with these trigger pulses. While circuit exerciser 14 may be capable of generating a test pattern having a rising edge of, say, 10 ps duration as shown in line "b" of FIG. 3, the impedance encountered over the path from input pin 48 to the active element 34 to be examined acts as a low-pass filter and introduces some indeterminate delay. That is, the rise time of the pulse at input port 50 of active element 38 may have a duration of, say, 50 ps as shown in line "c" of FIG. 3. While on-chip element 38 may be able to restore the rise time of the pulse at input port 52 of element 34 to 10 ps as shown in line "d" of FIG. 3, the intended placement of the rising edge is no longer assured.